Previously known methods for deinking of newsprint involve the production of a slurry of the paper in an aqueous alkaline solution containing sodium silicate, an anionic or nonionic surface active agent and a bleaching agent. The mixture is warmed up to 50.degree. - 70.degree. C with stirring to remove the ink from the paper in the form of a dispersion. Following this, the pulp can be separated either by (a) sieving, followed by rinsing and pressing, whereby considerable quantities of polluted water are obtained, or (b) by direct flotation of ink from the pulp by the addition of a soap, either before or after the pulping, and afterwards addition of a calcium salt to the mixture, whereby a calcium soap precipitates out and this together with the ink can be floated off. In this case there remains a mixture of paper pulp and clear water.
The problem with this known method is that large quantities of highly polluted water are obtained which are difficult to clean. This water usually has to be cleaned before it can be released into surface water. The costs for such cleaning are considerable.
The problem with the second method is that flotation with a calcium soap is not sufficiently selective, and a certain amount of pulp fibers are also floated off. Further, it has been discovered that ink flocs adhere to walls, pipes and scrapers of the deinking apparatus, and this necessitates closing down the equipment about one day a week for cleaning. Further, these ink flocs which become attached to the equipment can break loose and discolor the deinked pulp. Another problem with methods utilizing high pH-dispersions at high temperatures is that a discoloration (yellowing) of the pulp occurs and this necessitates the inclusion of relatively expensive chemical bleaching systems in the deinking process.
Newsprint inks are composed of two main types: so called letterpress ink and offset ink. Both types contain binders, which are organic compounds often with anionic groups, and an organic oil. As pigment particles carbon black is mainly used, with or without an organic pigment, also called toner. Colored printing inks contain principally organic pigments.